Note that arithmetic with floating point is full of surprises; due to precision
issues associativity of arithmetic operations typically do not hold. Also,
the presence of NaN is always something to look out for.

Prove that addition is not associative, even if we ignore NaN/Infinity values.
To do this, we use the predicate isFPPoint, which is true of a floating point
number (SFloat or SDouble) if it is neither NaN nor Infinity. (That is, it's a
representable point in the real-number line.)